Defects of vision
Long sight
- A long-sighted person can see objects a long distance away (they have good long sight) but can鈥檛 see objects a short distance away. A long-sighted person can read writing on a whiteboard in a classroom clearly but cannot read print in a book or newspaper sharply.
- Images of nearby objects (at 25cm from the eye) are formed behind the The light-sensitive area on the back of the eye. Light must be focused on the retina in order to see clearly.. The image is blurred.
- Long sight is due to the eyeball being too short, or the lens cannot be made thick enough by the ciliary muscles to focus the light rays on the retina. Long sight often occurs in older people as the ciliary muscles weaken with age.
Correcting for long sight
A convex lens corrects long-sightedness, allowing an image to focus on the retina
- Rays from a nearby object need to be converged more, to form the image on the retina.
- Long sight is corrected using a converging lens which starts to converge light rays from a nearby object before they enter the eye.
- Converging (convex) lenses are used in reading glasses.
Short sight
- A short-sighted person can see close objects clearly (they have good short sight), but they can鈥檛 see distant objects clearly. A short-sighted person can read a book clearly but cannot read writing on a whiteboard in a classroom or a car number plate at distance.
- The image of a distant object, say 2 m to 3 m from the eye, is formed just in front of the retina, causing it to appear blurred.
- This defect is due to the eyeball being too long or the ciliary muscles cannot make the lens thin enough.
Correcting for short sight
Concave lens cures short-sightedness
- Rays from a distant object need to be spread out, before they reach the lens.
- Short sight is corrected using a diverging lens which diverges the light rays from a distant object before they enter the eye.
- Diverging (concave) lenses are used in spectacles for distance viewing.